1. Field of the Invention
The present invention relates generally to continuously variable transmissions for use in automobiles, trucks, bicycles and other vehicles. More particularly, the present invention relates to a hardgeared continuously variable transmission capable of producing an infinite range of input speed to output speed ratios.
2. Description of Related Art
A transmission is used to convert a rotating input power source, such as that supplied by an automobile engine, to a desired output. The transmission establishes a ratio of input rotational speed to output rotational speed appropriate for a particular operating condition. For example, the transmission can alter the amount of output speed produced by a given input speed, or alter the amount of input speed required to produce a given output speed. In a conventional transmission, the conversion from input to output speed involves switching between a finite number of fixed gear ratios. A user selects one of the fixed gear ratios using a stick shift and clutch, or the switching can occur automatically, as in the case of an automatic transmission. In either case, however, the fixed gear ratios of conventional transmissions are inefficient in that an optimal input/output speed ratio cannot be continuously maintained over varying speed and power requirements. The ratio is restricted to one of the fixed ratios even though the same output speed, for example, could be produced with less input speed and therefore less engine power. This inefficiency means engines must work harder than necessary under many operating conditions, leading to increased engine wear and lower fuel economy.
Continuously variable transmissions (CVTs) address the inefficiency problem in conventional fixed gear ratio transmissions. In a CVT, the input/output speed ratio can be continuously varied to provide optimal ratios within a certain range. Infinitely variable transmissions (IVTs) are CVTs with an infinite speed ratio range, and can therefore provide an optimal speed ratio for all operating conditions. For example, an IVT allows zero output rotational speed with a non-zero rotating input source, and therefore need not disengage the input source to provide a neutral output. Unlike IVTs, CVTs must employ either a clutch mechanism or operate in conjunction with a planetary gear train to provide speed ratios outside the limited CVT speed ratio range.
For many years, manufacturers have attempted to design fuel efficient automobiles incorporating CVTs or IVTs that continuously adjust the speed ratio to maintain the optimal engine speed for a given operating requirement. Despite these efforts, the only commercial implementations currently available are belt driven CVT designs which typically include a continuous flexible belt riding on two pulleys. The speed ratio is varied by adjusting the effective diameter of the pulleys. However, belt driven CVTs such as these cannot accommodate the power requirements of larger vehicles. Their application is therefore limited to small automobiles and light industrial uses. Furthermore, belt driven CVTs suffer from reliability problems. The belts, whether made of rubber, metal or other materials, must be held under high tension to prevent slippage under varying operating conditions. The required high tension leads to excessive wear. U.S. Pat. No. 4,864,889 describes a belt driven CVT that is representative of the current practice.
Other IVT and CVT designs incorporate friction drives utilizing either traction or hydraulic mechanisms, although such systems are not commercially available. U.S. Pat. No. 4,693,134 describes a friction drive IVT designed for motor vehicle applications. This IVT employs traction rollers that contact a variable diameter disk to provide a continuously variable input/output speed ratio. A split torque arrangement is used to reduce the loads placed on the IVT elements. However, friction drive systems are typically unable to provide sufficient torque for large automobiles or relatively heavy industrial uses. In addition, the complexity of friction drive systems prevents their application in lighter weight vehicles such as bicycles. Friction drive IVTs and CVTs also suffer from reliability problems resulting from excessive wear.
Hardgeared designs have been developed in an attempt to overcome the reliability problems associated with belt and friction drive systems. In a hardgeared transmission, the primary means for converting input speed to output speed is hardgeared to the rotating input source. Such designs might also incorporate some form of belt or friction drive as a secondary conversion means not directly driven by the input source. U.S. Pat. No. 4,854,190 describes a hardgeared CVT employing a complex floating planetary gear train controlled by mechanical feedback loops and electrically engaged clutches. U.S. Pat. No. 4,983,151 describes a hardgeared IVT design using planetary gear trains to generate oscillating variable speed motions which are combined into an average output rotation by overriding one way clutches. Adjustment of the oscillating motion is used to control the input/output speed ratio. An infinitely variable ratio is produced by gearing together the oscillation output speed and the input speed, and then subtracting the input speed using a differential gear. Both of these hardgeared designs are highly complex and include a large number of gears and supporting mechanisms, leading to high manufacturing costs, additional reliability problems and limited transmission efficiency.
Friction drive and hardgeared CVT arrangements have also been applied to bicycles, as a replacement for conventional chain and sprocket transmissions. For example, U.S. Pat. No. 4,909,101 describes a bicycle CVT employing a sun gear, a plurality of planetary gears, an overriding clutch and an eccentric system. As the eccentricity is increased, the planetary gear superimposes a pulse drive on the sun gear that exponentially multiplies the drive rotation rate U.S. Pat. No. 4,854,191 describes a speed change mechanism for a bicycle that includes a planetary gear mechanism in which lock pawls are provided to control locking and unlocking of a plurality of sun gears. However, both of these approaches involve considerable manufacturing expense, and lead to weight and efficiency problems over conventional bicycle transmissions. Furthermore, conventionally designed bicycles cannot be easily retrofitted with these CVT designs. Instead, the bicycle must be custom designed to incorporate one of the currently available CVTs.
As is apparent from the above, there presently is a need for a simple, durable and highly efficient IVT that can improve the operating performance of a wide variety of vehicles including but not limited to automobiles, trucks and bicycles. The IVT should provide the reliability and performance advantages of a hardgeared transmission without the additional cost and complexity associated with currently available hardgeared designs. Furthermore, the IVT should be capable of providing sufficient torque to accommodate high power and torque applications, yet be simple enough to also accommodate lightweight low power applications, such as bicycles.